Britain is trying to weaken European proposals to make governments and companies cut their carbon emissions by 2020 to tackle global warming, the Guardian has learned. Leaked documents show Britain wants Brussels to offset more domestic carbon savings through investment in clean projects in the developing world.

The move would let firms and countries import more carbon credits to count against their pollution targets. It would allow Europe to make less effort to cut its pollution, while keeping it on course to meet an ambitious target of reducing carbon emissions by 20% by 2020.

The government's own calculations show the proposed change would allow Europe to emit an extra billion tonnes of CO2 from 2013-2020.

The move was condemned last night by environment campaigners, who accused the UK of trying to undermine efforts to get European industry to reduce emissions.

Caroline Lucas, MEP and leader of the Green party, said: "The British government is trying to buy its way out of climate change targets using unreliable credits from abroad. It shows how much of the political talk on climate is empty rhetoric, when you have the UK talking up the need for action on one hand, and carrying out this kind of irresponsible climate vandalism on the other."

The change is described in a discussion paper prepared by the UK on possible amendments to the EU climate and energy package, a copy of which has been obtained by the Guardian. Dated August 8, the paper says: "There are many good reasons why we should support increased access to project credits," and says the contribution of credits from schemes such as the UN's clean development mechanism (CDM) should be "limited to 50% of absolute effort".

In other words, it wants to allow half of the targeted carbon savings to be achieved through imported credits. The original EC proposal set the limit at about a quarter.

The UK position is confirmed in a separate briefing note sent to MEPs earlier this month.

The credits would be bought using the CDM or similar carbon markets set up under the Kyoto protocol.

Projects in the developing world that save carbon, such as technology to generate electricity from renewable sources, generate carbon credits, can then be bought by companies and used to notionally reduce their own emissions.

Campaigners say the carbon credits are often unreliable and are no substitute for direct reductions. A US report earlier this year claimed between a third and two thirds of CDM credits did not reflect genuine carbon savings.

Keith Allott of WWF said: "Europe has a responsibility to help developing countries move onto a low-carbon pathway, but this cannot come at the expense of high emissions at home. We need to move beyond the idea of offset Europe."

The proposal is the latest UK attempt to weaken the EC environmental package, which faces critical votes in Brussels next month. In July, the Guardian revealed how Britain wanted to block attempts to give renewable electricity sources priority access to electricity grids. The UK has also targeted the pledge to generate 20% of EU energy from renewable sources, with an attempt to make the target voluntary and to count projects abroad.